Environmental Impact of Radionuclides and Trace Elements in the Thorium Rich Fen Area in Norway
Miljøkonsekvenser knyttet til radionuklider og sporelementer i det thoriumrike Fensfeltet i Norge
Philosophiae Doctor (PhD) Thesis Jelena Mrdakovic Popic
Centre for Environmental Radioactivity Department of Environmental Sciences
Norwegian University of Life Sciences Ås 2014
Thesis number 2014:26 ISSN 1503-1667 ISBN 978-82-575-1196-8
PhD supervisors Professor Lindis Skipperud
Centre for Environmental Radioactivity Department of Environmental Sciences Norwegian University of Life Sciences P. O. Box 5003, 1430 Ås
Norway
Professor Brit Salbu
Centre for Environmental Radioactivity Department of Environmental Sciences Norwegian University of Life Sciences P. O. Box 5003, 1430 Ås
Norway
Professor Terje Strand Division for Science
The Research Council of Norway Stensberggata 26, N-0131 Oslo Norway
PhD Evaluation Committee Professor Peter Stegnar
Jožef Stefan International Postgraduate School (IPS) Jamova 39, 1000 Ljubljana
Slovenia
Dr. Per Roos, Senior Scientist
Center for Nuclear Technologies (DTU NUTECH) Technical University of Denmark
Roskilde Denmark
Committee Administrator: Associate Professor Elin Gjengedal Department of Environmental Sciences
Norwegian University of Life Sciences P. O. Box 5003, 1430 Ås
Norway
Acknowledgements
This thesis is the end of my journey in obtaining a doctorate degree. Work on my PhD thesis was financed by a grant from the Norwegian University of Life Sciences, following the participation in the Thorium Committee (2008). My PhD research work was kept on track with the support and encouragement of my supervisors, colleagues and family. Now, at the end, it is a pleasant task to express my gratitude to all who in different ways contributed to the success of this endeavor and made it an extraordinary experience for me.
Although environmental chemistry caught my interest early during my University education, my first encounter with environmental radioactivity was in 2005 when I was working in the Institute for Technology of Nuclear and other Mineral Raw Materials (Belgrade, Serbia). Work on materials for remediation of uranium- contaminated soil opened the door for my Norwegian academic story. At that door, I first met Lindis Skipperud who was my professor, supervisor and has been my friend for the last 6 years. I want to thank you Lindis, for providing me with the opportunity to work on this project and be a part of the Isotope Laboratory. Your help was immeasurable regarding all scientific guidance, discussions and comments. Thank you for all of the interesting field trips to Fen where I learned so much about natural ionizing radiation, and thank you also for allowing me to participate at different conferences where I heard many new things and met interesting scientists. And last but not least, thank you so much for all the personal support, care and thoughts you sent me in those moments when life was not easy for me.
I thank Professor Brit Salbu for her significant role in this project, for the knowledge of radiochemistry and radioecology I obtained from her, for her constructive scientific suggestions, valuable comments on manuscripts and my PhD thesis, as well as for participation in fieldwork. I also wish to acknowledge Professor Terje Strand for the data input on thorium in the Fen area, for the help in fieldwork, and the work on Paper I and the PhD thesis.
I am grateful to my colleagues Gjermund Strømman and Karl Andreas Jensen for their help and advices in ICP-MS analyses, Sondre Meland for multivariate statistics and Cato C. Szacinski Wendel for being so helpful in all aspects. Thank you Signe Dahl for all of the practical help during manuscript and thesis writing, but also for your personal care and support. I also wish to express my deepest gratitude to all other colleagues at the Isotope Laboratory who were always so cooperative and friendly. It was great working with all of you!
I would like to thank my family for their love, patience and support. Thank you my dear sister Marija, for always being there whenever I needed you, thank you for proofreading my papers and this thesis, and thank you for all of your advices while I was working on my PhD thesis and for looking after my baby when I had to work. I also wish to thank my mother for the comfort she provided during different stages of my life.
Finally, I would like to thank my husband Nenad, for being true and a great support, for his unconditional love during these past several years that have not been an easy ride, both academically and personally. Our daughter Nadja has brought much joy to our lives and I am really grateful to the universe for that!
Jelena Mrdaković Popić, Ås, December 2013
Summary
The Fen Complex which is situated in the south of Norway, represents a magmatic bedrock area enriched in thorium (Th), iron (Fe), niobium (Nb) and rare earth elements (REE), and is well known for the elevated levels of natural ionizing radiation. This area has been of public interest from the 17th century when Fe mining started in the central wooded zone of the Fen Complex. Intensive mining of Fe continued until the 20th century, while mining and the production of ferro-niobium were conducted at the site Søve in the western part of the Complex in 1950s.
Recently, intensive focus has been directed to the estimated large quantities of Th and REE ores, their value and possibility for future use, but also to environmental issues linked to the legacy enhanced naturally occurring radioactive materials (NORM) in the area. Many studies investigating different aspects of specific bedrock geology, as well as human health risk related to elevated ionizing radiation levels in the area, have been published. Still, no comprehensive investigation of different environmental compartments and radionuclides impact on biota at both legacy enhanced and undisturbed NORM sites in the Fen Complex has been undertaken.
The present work was initiated as an integrated ecological and human impact assessment whose main objectives were to assess the possible radionuclide and trace elements contamination of the Fen Complex environment, impact on biota and radiation doses to humans due to outdoor radiation exposure. The Fen Complex area which is comprised of both legacy NORM and undisturbed 232Th-rich sites served as a natural laboratory where environmental compartments and biota could be investigated in the natural state, under realistic conditions. With respect to the specific Fen area and previously published data, the main focus of this work was on radionuclides such as 232Th and uranium (238U) and their progenies, as well on trace elements such as arsenic (As), chromium (Cr), cadmium (Cd) and lead (Pb). The generated data provided information on the current environmental status at both undisturbed and legacy NORM sites in the area, and could be used in planning of eventual remediation activities or for future monitoring.
To assess the impact of radionuclides and trace elements on the ecosystem and humans, information was needed regarding the characterization, mobility and biological uptake of radionuclides and trace elements, as well as their different exposure pathways. These aspects were studied and presented in five scientific papers on which this thesis is based on.
The first paper (Paper I) describes the initial screening of the Fen Complex. It included the analysis of radionuclides and trace elements in samples of soil, rock, water and plants. The high gamma dose rates in outdoor air were recorded. Based on the obtained data, radionuclides in soil were inhomogeneously distributed and ˝hot spots˝ with high levels of radionuclides (up to about 7000 Bq/kg 232Th and 150 Bq/kg
238U) and elevated gamma dose rates (up to 10 µGy/h) were identified. ˝Hot spots˝
were observed within legacy NORM (former mining) sites, and also in some undisturbed surrounding 232Th-rich sites. The initial ERICA impact assessment demonstrated that dose rates for certain terrestrial organisms were higher than the adopted screening level (10 µGy/h), suggesting the need for more refined analysis. In addition to the previously published literature, the data presented in Paper I assisted in defining future directions of investigation, the choice of sites for more detailed surveys, biota selection and in defining aspects of human exposure that would be investigated.
Binding mechanisms in soil determine the potential mobility and bioavailability of radionuclides and trace elements, and soil fractionation is therefore essential for assessing their behavior in the environment. In the present work, mobility analysis of the investigated elements, based on the results of sequential extractions, was performed (Paper II). Soil fractionation showed that the majority of 232Th and As were irreversibly bound in the soil as they were only leached by concentrated HNO3
at elevated temperature. With respect to 232Th, the result was in accordance with its chemical nature, established low mobility and tendency to be tightly bound in soil fractions. Uranium and trace elements (Cr, Cd, Pb, Ni, Cu and Zn) were found to be potentially more mobile and associated with pH-sensitive soil phases, redox-sensitive amorphous soil phases and organic soil compounds. Multivariate statistical data analysis provided the link between soil chemical and physical parameters and output data from the sequential extractions. Further mobility investigation was performed by determining the distribution coefficients (Kd). The Kd (232Th) and Kd (238U) suggested elevated dissolution and mobility at legacy NORM sites, especially at decommissioned Nb mining site (346 and 100 L/kg for 232Th and 238U, respectively), while higher sorption of radionuclides was demonstrated at undisturbed 232Th-rich site (10672 and 506 L/kg for 232Th and 238U, respectively).
Earthworms were chosen as biota representative for a detailed analysis of radionuclide and trace element uptake and chronic exposure to radiation (Paper III).
Tissues of four different earthworm species, including epigeic and endogeic species, were analyzed and the results were linked to total soil concentrations, bioavailable or extractable soil fractions, and root and litter concentrations in order to predict the
favorable environmental pool for uptake. Variability in individual tissue concentrations of radionuclides was observed to be high as previously demonstrated in scientific literature for heavy metals in earthworms. Differences in uptake between four investigated earthworm species, but also between species collected at legacy NORM and undisturbed 232Th-rich sites were demonstrated. Higher transfer was observed for 238U (TF = 0.09 – 0.25) than for 232Th (TF = 0.03 – 0.08). Radiological dose rates (2.2 – 11.9 µGy/h), obtained by ERICA modeling, were higher than those generally experienced by terrestrial organisms (0.01 – 0.7 µGy/h) in the soil with background radionuclide concentrations. However, no radiation risk could be predicted since the obtained doses were much lower than the internationally (IAEA, 1992; UNSCEAR 2008; US DOE 2002) adopted levels of ionizing radiation below which no measurable population effects would occur (40 and 400 µGy/h).
Further study of biota exposure was performed on nine wild plant species. A wide range of plants was included, with different uptake modes taken into account, and including both roots and aboveground plant parts (Paper IV). Plant tissue concentrations of radionuclides were only slightly enhanced (up to 50 and 5 Bq/kg of
232Th and 238U, respectively), and comparable to the values reported in literature. The levels of trace elements were within the reference range for plants. Roots appeared to be a natural barrier to radionuclide entry into plants. This is illustrated by the finding that the activity concentrations were higher by a factor of 25 in roots than in the aboveground plant parts. Thus, the transfer factors for plants were actually lower (4·10-5 – 1·10-2 for 232Th and 1·10-4 – 4·10-2 for 238U) than expected from the observed total soil concentrations (about 16000 Bq/kg 232Th and 900 Bq/kg 238U). Based on the ERICA calculation, dose rates up to 23 µGy/h (in moss and lichen) were predicted.
Previous studies on human health risk in the Fen area have demonstrated that the annual exposure doses were among the highest in Europe (about 14 mSv). The majority of these investigations focused on the doses of indoor gamma radiation and radon (222Rn), as well as on the doses received via ingestion of food and water. In the conclusions presented in several papers, the need for investigating the contribution of outdoor exposure to total exposure doses, as well for measuring the 220Rn concentration in air was highlighted. This information focused our investigation into outdoor 220Rn, 222Rn and terrestrial gamma radiation (Paper V). Compared to the world average, high outdoor gamma dose rates (about 10 µGy/h), high 220Rn (up to 5000 Bq/m3) and moderate 222Rn (up to 200 Bq/m3) concentrations in the air were recorded in the Fen area. Levels of these parameters correlated with the distribution of radionuclides in the bedrock. Due to the high uncertainty when 220Rn is used to calculate the exposure doses, the annual outdoor doses (0.10 – 1.54 mSv) were
obtained by summarizing the doses from terrestrial gamma radiation and 222Rn doses.
However, when variations in the exposure times and the 220Rn dose (calculated using the equilibrium factor F from an earlier study in the Fen) are accounted for, an increase in the annual outdoor doses for over 10 mSv for at least some people can be expected.
Based on the overall results, the high concentrations of radionuclides in the soil, the high levels of terrestrial gamma radiation and the high outdoor levels of 220Rn and
222Rn were observed at both legacy NORM and undisturbed 232Th-rich sites in the Fen Complex. However, due to the relatively low mobility of radionuclide 232Th, no significant transport into investigated biota, such as earthworms and plants, was demonstrated so that low radiation doses and no elevated risk were predicted using the ERICA tool. However, the question of synergistic, additive or antagonistic actions of radionuclides and trace elements in biota of the Fen Complex remains. In the analysis of human outdoor exposure, high terrestrial gamma radiation was demonstrated to have a major impact on the magnitude of the received doses. Outdoor exposure doses higher than 10 mSv/y, for at least a certain group of people under specific exposure scenarios, were estimated as possible. Due to the high outdoor 220Rn concentrations, the contribution of 220Rn to the total dose of outdoor exposure could not be excluded, although quantification of such a contribution with the present 220Rn data set is connected to a large uncertainty. With respect to the current Norwegian legislation, an intervention should be considered at the legacy NORM site Søve.
Although, there is little that could be done to change the doses at undisturbed high radioactivity sites, it would be reasonable, where possible, to avoid house construction, to restrict the time spent for recreation and to limit the use of the materials with elevated radioactivity.
Sammendrag
Fensfeltet, lokalisert i Sør Norge, representer et magmatisk berggrunnsområde beriket med thorium (Th), jern (Fe), niob (Nb) og sjeldne jordelementer (REE). Det er velkjent for forhøyet nivå av naturlig ioniserende stråling. Fensfeltet har vært av offentlig interesse siden 1600-tallet da gruvedriften av Fe begynte i sentral skogkledd sone. Det ble drevet intensiv gruvedrift av Fe frem til 1900-tallet, mens gruvedrift og produksjon av ferro-niob ble gjennomført på 1950-tallet i Søve gruver, i den vestlige delen av komplekset. Nylig ble fokus igjen rettet mot Fensfeltet og store estimerte konsentrasjoner av Th og REE, deres verdi og muligheter for fremtidig bruk, samt mot miljøproblematikk knyttet til historiske NORM områder som eksisterer i området. Det har blitt publisert flere studier som forsket på forskjellige aspekter av spesifikk berggrunnsgeologi, samt helserisiko for mennesker relatert til forhøyede nivåer av forhøyet ioniserende stråling i område. Fortsatt har ingen omfattende forskning på forskjellige miljøaspekter og påvirkning av planter i verken tidligere gruveområder eller uforstyrrede NORM områder i Fensfeltet blitt gjennomført.
Dette arbeidet ble initiert som en integrert studie for miljøvurdering. Hovedmålet var å få til risikovurdering av av Fensfeltet fra radioaktivitet og metaller blant annet ved å se på påvirkning av biota og stråledoser for mennesker gjennom utendørs eksponering. Fensfeltet, bestående av både tidligere gruveområder med NORM og uforstyrrede områder rike på 232Th, ble brukt som naturlige laboratorier hvor jord, vann, luft og biota kunne undersøkes under realistiske forhold. Basert på tidligere publiserte data og opplysninger om området, ble hovedfokus rettet mot radionuklider som 232Th, uran (238U) og deres døtre samt sporelementer som arsen (As), krom (Cr), kadmium (Cd) og bly (Pb). Genererte data ga opplysninger om nåværende miljøstatus i både uforstyrrede og historiske NORM områder. Disse dataene kan bli brukt til planlegging av eventuelle aktiviteter for utbedring og fremtidig overvåkning. For å vurdere radionukliderelatert påvirkning av økosystemet og mennesker, var det nødvendig å gjøre karakterisering av forskjellige prøver, undersøke mobilitet og biologisk opptak av radionuklider og sporelementer, samt deres eksponeringsvei og påvirkning av biota og mennesker. Disse aspektene ble studert og presentert i fem vitenskapelige artikler som denne doktoravhandlingen er basert på.
I den første artikkelen (Paper I) ble det gjort en innledende undersøkelse av Fensfeltet. Dette inkluderte analyser av radionuklider og sporelementer i jord, stein, vann og planter. Nivåer av gammastråling i utendørs luft ble registrert. Innhentede data viste at radionuklider i jord var heterogent fordelt og ˝hotspots˝ med høye nivåer av radionuklider (opp til 7000 Bq/kg 232Th og 150 Bq/kg 238U) og høye nivåer av
gammastråling (opp til 10 µGy/h) ble identifisert. ˝Hotspots˝ med forhøyede verdier av 220Rn ble observert i både tidligere gruveområder med NORM og uforstyrrede
232Th-rike områder. ERICA-modellert vurdering viste at doser for visse organismer var høyere enn vedtatt screening nivå på 10 µGy/h, noe som tyder på behov for mer sofistikert analyse. Sammen med tidligere publiserte data, var dataene presentert i Artikkel I avgjørende for planleggingen av videre forskning, valg av områder for mer detaljert undersøkelse, valg av biota og definisjon av aspekter for menneskelig eksponering som ble undersøkt videre.
Den kjemiske spesieringen forteller noe om mobilitet og biotilgjengelighet av radionuklider og sporelementer, og er derfor avgjørende for å vurdere oppførselen og skjebnen i miljøet. I dette arbeidet ble det gjennomført jord- og vannfraksjonering og mobilitetsanalyse, i tillegg til måling av totale konsentrasjoner (Artikkel II).
Jordfraksjonering ved hjelp av sekvensiell ekstraksjon, viste at 232Th og As var irreversibelt bundet i flertallet av jordprøvene. Fordelingen av 232Th var i samsvar med dets kjemiske natur og kjente lave mobilitet, samt tendensen til å være fast bundet i krystalliserte jordfraksjoner. Uran og sporelementer (Cr, Cd, Pb, Ni, Cu og Zn) viste seg å være mer potensielt mobile og assosiert med pH-sensitive jordfaser, redox-sensitive amorfe jordfaser og jord-organisk stoff. Multivariat statistiske analyser av dataene ga oss leddet mellom kjemiske og fysiske parametere i jord og resultater av sekvensiell ekstraksjon. Den videre undersøkelsen av mobilitet ble gjort med analyse og beregning av fordelingskoeffisienten (Kd). De beregnede verdiene viste forhøyet mobilitet og mulighet for transport videre i miljøet ved tidligere gruveområder, spesielt ved gruveområdet for Nb ved Søve (346 and 100 L/kg for henholdsvis 232Th and 238U), mens høy adsorpsjon og dermed lavere mobilitet, ble demonstrert ved det naturlig 232Th-rike området Rullekoll (10672 and 506 L/kg for henholdsvis 232Th and 238U).
Meitemark ble valgt som biota egnet for detaljert analyse av radionuklider og sporelementer, samt opptak og mottak av kroniske stråledoser (Artikkel III). Vev fra forskjellige arter, inkludert epigeiske og endogeiske meitemark fra Fensfeltet, ble analysert for radionuklider og sporelementer. Resultatene ble koblet til totale jordkonsentrasjoner, biotilgjengelige jordkonsentrasjoner og konsentrasjoner i røtter for å kunne forutsi foretrukket medium for opptak. Variasjon i individuelle vevskonsentrasjoner ble observert i likhet med tidligere publiserte data i internasjonal litteratur for tungmetaller i meitemark. Det ble påvist forskjeller i opptak mellom forskjellige arter, men også forskjeller i undersøkte områder. En høyere overføring av
238U (TF = 0,09 – 0,25) enn av 232Th (TF = 0,03 – 0,08) ble demonstrert. ERICA modellering påviste høyere radiologiske doser (2,2 – 3,9 µGy/h) enn gjennomsnittlige
doser (0,01 – 0,7 µGy/h) for jordorganismer som lever i miljøer med lave nivåer av radionuklider i bakgrunn. Likevel kunne man ikke forutsi noen økt strålerisiko, siden beregnede doser fortsatt var mye lavere enn internasjonalt vedtatte nivåer av ioniserende stråling som kan produsere biologiske effekter (40 og 400 µGy/h).
Videre analyse av biotaeksponering ble gjort for et bredt spekter av samlede planter fra Fensfeltet, inkludert både røtter og hele planter av ni forskjellige plantearter (Artikkel IV). Konsentrasjon av radionuklider i plantevev var kun lett forhøyet (opp til 50 Bq/kg 232Th og opp til 5 Bq/kg 238U) og i godt samsvar med verdier sett i litteraturen. Nivåer av sporelementer var innenfor referanseområdet for planter. Røtter virket å representere en naturlig barriere for opptak av radionuklider i planter siden aktivitetskonsentrasjoner i disse var opptil 25 ganger høyere enn i plantedelene over jorda. Generelt sett var overføringsfaktorene for planter (4·10-5 til 1·10-2 for 232Th and 1·10-4 til 4·10-2 for 238U) lavere enn forventet ut i fra konsentrasjonene i jord (16000 Bq/kg 232Th og 900 Bq/kg 238U). Basert på ERICA-modellering ble det påvist doser opp til 10 µGy/h (i mose og lav). Tilsvarende som for meitemark, ble konklusjonen at ingen risiko kunne estimeres og det var ikke nødvendig med videre analyser.
Tidligere studier av helserisiko for mennesker i Fensfeltet har vist eksponeringsdoser fra gammastråling som er blant de høyeste i Europa (rundt 14 mSv). Majoriteten av disse studiene fokuserte på innendørs gammastråling og radon (222Rn) doser samt på doser inntatt gjennom mat og vann. I konklusjonen til flere artikler ble det understreket et behov for undersøkelse av bidrag fra utendørs eksponering i beregninger av totale eksponeringsdoser, samt måling av 220Rn og 222Rn konsentrasjoner i luft. Dette rettet vår forskning mot utendørs 220Rn og 222Rn og jordisk gamma stråling (Artikkel V). Sammenlignet med verdensgjennomsnittet ble det i Fensfeltet registrert høye utendørs gammadoser (opp til 10 µGy/h), høye 220Rn- (5000 Bq/m3) og moderate 222Rn- (200 Bq/m3) konsentrasjoner i luft. Nivåene av disse parameterne var korrelert med distribusjonen av radionuklider (Th og U) i berggrunn. På grunn av stor usikkerhet ved bruk av 220Rn konsentrasjon i luft for å beregne eksponeringsdoser, ble de totale årlige utendørsdosene (0,10 – 1,54 mSv) gitt som summen av doser fra gamma stråling og doser fra 222Rn. Ved variasjon av eksponeringstid ved beregning av 220Rn-doser (ved hjelp av likevektsfaktor Fi fra tidligere studier i Fensfeltet) ville man få høyere årlige utendørsdoser, over 10 mSv, i hvert fall for noen grupper mennesker.
Basert på de samlede resultatene ble det målt høye konsentrasjoner av radionuklider i jord, samt høy jordisk gammastråling og utendørsnivåer av Rn i både tidligere gruveområder og i uforstyrrede 232Th-rike områder i Fensfeltet. På tross av dette ble det, grunnet relativt lav mobilitet av radionukliden 232Th, ikke påvist noen signifikant
transport til undersøkte biota (planter og meitemark). Derfor ble det beregnet lave eksponeringsdoser ved bruk av ERICA-programmet og ingen økt risiko kunne bli estimert. På tross av dette, er spørsmålet om synergistisk, additiv eller antagonistisk virkning av radionuklider og sporelementer i biota fortsatt åpent. I analysen av utendørs eksponering av mennesker, hadde høy gamma stråling hoved-innvirkningen på de årlige strålingsdosene. På grunn av høye 220Rn konsentrasjoner, kan ikke et bidrag fra 220Rn til de totale årlige doser kunne utelukkes, selv om kvantifisering av årlige doser (>10 mSv) er koblet til stor usikkerhet og derfor ikke anbefalt av internasjonale organisasjoner. I følge den nåværende Norske Forurensningsloven, burde man vurdere tiltak i det tidligere gruveområdet Søve. Generelt er det lite som kan gjøres på uforstyrrede områder med naturlige høye nivåer av radioaktivitet, men det kunne vært fornuftig å begrense bygging av hus, bruk av radioaktive materialer fra området og begrense tilgang til områdene med påvist høy radioaktivitet.
Summary of publications
This thesis is based on the following papers which are referred to in the text by their Roman numerals.
Paper I Mrdakovic Popic, J., Salbu, B., Strand, T., Skipperud, L. (2011).
Assessment of radionuclide and metal contamination in a thorium rich area in Norway. Journal of Environmental Monitoring 13, 1730-1738.
Paper II Mrdakovic Popic, J., Meland, S., Salbu, B., Skipperud, L. (2014). Mobility of radionuclides and trace elements in soils from legacy NORM and undisturbed 232Th-rich sites. Environmental Science: Impacts and Processes, in press, DOI: 10.1039/C3EM00569K.
Paper III Mrdakovic Popic, J., Salbu, B., Skipperud, L. (2012). Ecological transfer of radionuclides and metals to free-living earthworm species in natural habitats rich in NORM. Science of the Total Environment 414, 167-176.
Paper IV Mrdakovic Popic, J., Oughton, D., Salbu, B., Skipperud L. Transfer of radionuclides (232Th, 238U) to wild forest flora species in a thorium rich area, Manuscript in preparation.
Paper V Mrdakovic Popic, J., Bhatt, C.R., Salbu, B., Skipperud, L. (2012). Outdoor
220Rn, 222Rn and terrestrial gamma radiation levels: investigation study in the thorium rich Fen Complex, Norway. Journal of Environmental Monitoring 14, 193-201.
List of Acronyms and Codes
ALARA ˝As Low As Reasonable Achievable˝ radiation safety principle BSS Basic Safety Standards
CR Concentration Ratio
DCC Dose Conversion Coefficient
EEC Equilibrium Equivalent Concentration
Eh Redox Potential
ENRA Enhanced Natural Radiation Area ERA Environmental Risk Assessment
ERICA Environmental Risk from Ionizing Contaminants:
Assessment and Management
FASSET Framework for Assessment of Environmental Impact
F Equilibrium Factor
FWHM Full Width at Half Maximum GMC Geiger Müller Counter GPS Global Positioning System HCl Hydrochloric Acid HMM High Molecular Mass
HNO3 Nitric Acid
H2O2 Hydrogen Peroxide
HPGe High Purity Germanium Detector HTGR High Temperature Gas Cooled Reactor IAEA International Atomic Energy Agency
ICP-MS Inductively Coupled Plasma Mass Spectrometry
ICP-OES Inductively Coupled Plasma Optical Emission Spectrometry ICRP International Commission on Radiological Protection JSI Jožef Stefan Institute, Ljubljana, Slovenia
Kd Soil/Solution Distribution (partition) Coefficient LMFBR Liquid Metal cooled Fast Breeder Reactors
LMM Low Molecular Mass
LNT Linear No Threshold LOD Limit of Detection LOQ Limit of Quantification
LWR Light Water Reactor MSBR Molten Salt Breeder Reactor NH4OAc Ammonium Acetate
NH2OH · HCl Hydroxylamine Hydrochloride
NIRS National Institute of Radiological Science, Chiba, Japan NOR Naturally Occurring Radionuclides
NORM Naturally Occurring Radioactive Material NRPA Norwegian Radiation Protection Authority PCA Principle Component Analysis
pH Negative Logarithm of the Activity of the Hydronium Ion PHWR Pressurized Heavy Water Reactor
PIPS Passive Implanted Planar Silicon PNEDR Predicted No Effect Dose Rate REE Rare Earth Elements
RQ Risk Quotient
RSD Relative Standard Deviation SSNT Solid State Nuclear Tracks
SV Screening Value
TENORM Technologically Enhanced Naturally Occurring Radioactive Material
TF Transfer Factor
ThO2 Thorium Oxide
TLD Thermo Luminescent Dosimeter UMB (NMBU) Norwegian University of Life Sciences
UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation
UO2 Uranium Oxide
UO22+ Uranyl Ion
USGS Unites States Geological Surveys
US EPA United States Environmental Protection Agency
WHO World Health Organization
Table of contents
1 Introduction ... 1
1.1 Naturally occurring ionizing radiation ... 1
1.2 The thorium rich Fen Complex in Norway ... 2
1.3 Rationale, hypothesis and objectives of the thesis ... 5
2 Thesis background ... 8
2.1 Naturally occurring radioactive material (NORM) ... 8
2.1.1 Thorium (Th) ... 10
2.1.2 Uranium (U) ... 14
2.1.3 Radon (Rn) and Rn daughters ... 17
2.1.4 Gamma radiation ... 19
2.1.5 Interaction of ionizing radiation with biological tissue ... 20
2.2 Multiple stressors – radionuclides and trace elements... 21
2.3 Mobilization and ecosystem transfer ... 22
2.3.1 Sequential extraction ... 23
2.3.2 Distribution coefficients ... 24
2.3.3 Transfer factors ... 25
2.4 Impact of legacy and naturally enhanced radiation on humans and the environment ... 26
2.4.1 Ecological impact assessment... 27
2.4.2 Human impact assessment ... 29
3 Methodological approach ... 33
3.1 Planning and problem formulation ... 33
3.1.1 Time frame of the assessment... 33
3.1.2 Site description ... 33
3.1.3 Radionuclides and trace elements considered ... 37
3.1.4 Organisms considered - biota and human assessment ... 37
3.1.5 Screening criteria ... 38
3.2 Sampling, measurement and analysis ... 39
3.2.1 In situ measurement and sampling ... 40
3.2.2 Sample preparation ... 42
3.2.3 Analytical procedures ... 43
3.2.4 Measurement techniques ... 45
3.3 Data analysis ... 50
3.3.1 Used parameters and equations ... 50
3.3.2 Statistical analysis ... 53
4 Summary of scientific papers ... 54
4.1 Paper I – Characterization of contaminants in different environmental compartments in the Fen Complex ... 54
4.2 Paper II – Mobility analysis of radionuclides and trace elements in the Fen Complex soil ... 55
4.3 Paper III – Biota assessment: Radionuclide and trace element transfer from soil to free-living earthworm species ... 56
4.4 Paper IV – Biota assessment: Radionuclide transfer from soil to wild plant species ... 57
4.5 Paper V – Estimation of possible human outdoor exposure doses in the Fen Complex area ... 58
5 Results and discussion ... 59
5.1 Quality of the data ... 59
5.2 Contamination status of the Fen Complex environment ... 61
5.2.1 Outdoor terrestrial gamma dose rates and 220Rn, 222Rn concentrations ... 61
5.2.2 Radionuclides and trace elements in soil ... 63
5.2.3 Radionuclides and trace elements in water ... 67
5.2.4 Radionuclides and trace elements in biota ... 68
5.3 Exposure of biota ... 70
5.3.1 Transfer factors ... 71
5.3.2 Exposure dose rates for terrestrial organisms ... 71
5.4 Human outdoor radiation exposure ... 74
5.5 Uncertainties in the assessments ... 79
6 Conclusions ... 83
7 References ... 85
Paper I Paper II Paper III Paper IV Paper V
Papers I - V have individual page numbers
1 Introduction
The present study focuses on possible environmental impacts due to increased concentrations of radionuclides and trace elements in the Fen Complex which is situated in the Telemark county, southeastern Norway. Study sites, containing both undisturbed naturally occurring radioactive materials (NORM) and legacy NORM from past mining activities, represented convenient environmental laboratories to study radionuclides and associated elements and their speciation, distribution, impact on biota and humans. Different environmental compartments, i.e., air, soil, water and selected biota were studied. The approaches employed to assess the radionuclides and trace elements contamination and its impact on the environment are presented in the thesis.
An introduction to the Fen Complex area and its naturally occurring radioactive materials, the thesis hypothesis and objectives are given in Chapter 1. The theoretical background, the investigated parameters and approaches used to assess the ecological and human impacts are presented in Chapter 2. Problem formulation, identification of potential hazards, description of study sites, methodologies and measurement techniques are provided in Chapter 3. The obtained results, discussion and evaluation of the impacts based on the applied methodologies are presented in Chapters 4 and 5.
The conclusions are in Chapter 6.
1.1 Naturally occurring ionizing radiation
Naturally occurring ionizing radiation, in the form of cosmic and terrestrial radiation, has been ubiquitously present in the environment since the Earth’s origin. Exposure to radionuclides from the Earth’s crust, together with cosmic rays and cosmogenic radionuclides, accounts for the greatest part of the contribution to the annual human radiation dose of an average individual (Betti et al., 2004; MARINA, 2002;
UNSCEAR, 2000, 2008a). Exposure to uranium (238U), thorium (232Th) and their progenies (all of the members of decay series are in Figure 4, section 2.1.1.), 235U and potassium (40K) generate the dose received from terrestrial radiation sources. When present in the body, these radionuclides irradiate an organism internally via alpha and beta particles and gamma rays; externally they irradiate with gamma radiation.
At background concentrations, the naturally occurring radionuclides (NOR) in the
238U, 235U, and 232Th series, including radon (222Rn), contribute to over 80 % of the background radiation to which all humans are continuously exposed (UNSCEAR,
2000, 2008a). The vast majority of the world population receives annual doses around of 2.4 mSv, the average global exposure. More than about 98 % of the population receives annual doses that are lower than 5 mSv, and about 99 % of the population doses lower than 7 mSv. However, there are inhabited areas of the world, so called enhanced natural radiation areas (ENRA), where the average annual exposure doses from natural sources are above 10 mSv (UNSCEAR, 2000, 2008a). Since natural radiation is usually a far more significant contributor to the annual exposure dose than man-made radiation, the dose from naturally occurring radiation represents a baseline value for an annual radiation dose, to which other exposure doses are added under specific circumstances (e.g., due to medical reasons, nuclear accidental situations).
1.2 The thorium rich Fen Complex in Norway
The Fen Complex is located in the Telemark County in southeastern Norway. It is an intrusive complex of alkaline and carbonatite rocks. The central circular intrusive of about 5 kilometers diameter represents a cross-section of the feeder pipe of a Precambrian gneiss volcano. This volcano was active 580 million years ago (Barth and Ramberg, 1966). The area was first described by Brøgger (1921), and since then it has acquired fame in the geological society. Brøgger (1921), who discovered that carbonates in the Fen Complex had a magmatic origin, introduced the term
˝carbonatite˝ for carbonate rocks of an apparent magmatic origin and named different specific rocks after localities in the Fen area (Heincke et al., 2008). So far, different aspects of the geology of the Fen, including alkali-metasomatism of the country rock (˝fenitisation˝), have been studied in great detail by many authors (e.g., Andersen 1984; Bergstøl and Svinndal, 1960; Dahlgren, 1983; Dahlgren, 1987; Sæther, 1957).
The fact that the whole Complex was an active volcano implies variable depths for different ores, and the presence of heterogeneous rocks, with mixtures of many minerals and a variety of elements (Eriksen, 2012). Among the most abundant rock types of the Fen Complex area are søvite, rauhaugite, rødbergite, damtjernite and fenite (Figure 1).
Thorium ore resources in the Fen Complex have been estimated to be among the world’s largest, with an estimated volume of 170 000 tonnes, which are referred to as the reserve, and 150 000 tonnes, referred to as the reserve base (US GS, 2007).
However, knowledge of the geological environments and 232Th-enriched minerals in Norway is mainly based on the geological surveys conducted during two periods, after Second World War up to the 1960s, and from 1975 to 1985 (Thorium
Committee, 2008). In these surveys, Th was not a primary exploration target and resources were estimated in relation to U and rare earth element (REE) resources.
Figure 1. Geological cart of the Fen Complex in Telemark County (Ramberg et al., 2007).
Thus, the uncertainty about the exact quantities of 232Th has been highlighted (Thorium Committee, 2008). According to Berg et al. (2012), 232Th resources in the Fen Complex could even reach 675 000 tonnes. As such, these resources could represent potential energy up to 120 times larger than the oil extracted to date by Norway, plus that of the remaining oil reserves (Berg et al., 2012). A need for further investigations to obtain accurate data on the volume size of 232Th resources has been highlighted (Lindahl, 2007).
With respect to potential 232Th extraction, rødbergite is the most interesting rock type, containing the highest concentrations of 232Th (up to 0.4 wt %). However, 232Th has been found to be associated in small mineral grains (Figure 2). Separation of very fine-grained minerals and obtaining satisfactory enrichment are currently the main challenges for industrial exploitation since ore must be milled to fines <50 μm to extract 232Th-bearing minerals (Berg et al., 2012; Dahlgren, 2012; Eriksen, 2012).
Advanced methods for mineral separation should be adapted and utilized for these specific deposits (Thorium Committee, 2008).
Apart from 232Th, the Fen Complex has for a long time been of interest because of other mining possibilities. The iron (Fe) mines in the Fen area (˝Fen jerngruver˝ in Norwegian), located in the eastern parts of the Complex, are rich in rødbergite rock
and have been operating from 1657 to 1927. Reliable data on the mined quantities and produced Fe are not available. Open pits and parts of underground mining tunnels from the Fe production period, as well as the waste area on the slope of the Lake Nordsjø, are still visible.
Figure 2. a) Photo image of rødbergite (J.M.Popic) b) EM - inclusions of monazite and thorianite seen on scale 20 µm (Dahlgren, 2012) c) thorianite better seen on scale 10 µm (Berg et al., 2012).
At the mining site Søve (˝Søve gruver˝ in Norwegian) which was in operation from 1953 to 1965, niobium (Nb) was extracted. The main Nb-bearing mineral, pyrochlore, was identified already in 1918 in the søvite rocks. The average grade was about 0.35- 0.4 % Nb2O5. During the production period, Nb was extracted from the mineral søvite on a commercial basis. Waste material from the production of ferro-niobium was dumped as an aluminum-rich slag at a small hill nearby. The slag included about 570 tonnes which were covered and sealed with marine clays afterwards. In a remediation action conducted after the mine decommission, site Søve was covered with sand layer. Recent measurements have shown enhanced gamma radiation dose rates, and elevated Th and U concentrations, due to disturbance of protective sand layer and mixing with contaminated soil (IFE, 2006; NGI-UMB, 2010). Currently, a mechanical engineering firm is located at this former mining site, while the main hazard areas, including a sludge disposal site, wash house and slag heap (NGI-UMB, 2010) are freely accessible.
In addition to Fe and Nb mining, the investigation of REE in the Fen Complex has been in progress in last several decades and has actually revived interest recently. The interest in studying the relationship between geological formations and natural radiation in the area began in 1970-80s (Dahlgren, 1983; Stranden, 1982; Stranden and Strand, 1986; Svinndal, 1973). Dahlgren (1983) undertook the first gamma ray mapping of the terrain and demonstrated high terrestrial dose rates of gamma radiation over the whole area of the Fen Complex. Since health issues associated with
enhanced natural radiation have been of substantial interest for the Fen population, many studies have been directed to risk estimation and radiological protection of humans. The radiological and epidemiological impacts of the Fen mining activities, evaluated by Stranden (1982), showed that miners received an annual dose equivalent of 150 mSv, which is much higher than the occupational dose limit of 20 mSv/y for radiation workers. Furthermore, the indoor 222Rn concentrations in Fen area dwellings, the magnitude of indoor gamma radiation and their dependence on the geological formations of the terrain were also measured and discussed (Smethurst et al., 2006; Solli et al., 1985; Stranden, 1982; Sundal and Strand, 2004). The annual radiation dose to which the population in the Fen area was exposed was estimated to be higher (14 mSv) than the average radiation dose in Norway (2.9 mSv), due to elevated gamma radiation and elevated concentrations of 222Rn daughters (Stranden, 1982; Stranden, 1984; Stranden and Strand, 1986; Sundal and Strand, 2004). The most recent airborne gamma spectrometry mapping that relates indoor 222Rn concentrations to geological parameters (Heincke et al., 2008), has revealed elevated
232Th and moderate 238U concentrations in carbonatite rocks. Positive correlations between local 238U concentrations and indoor 222Rn concentrations were observed.
1.3 Rationale, hypotheses and objectives of the thesis
The rationale behind the current environmental assessment project was based on several points:
• According to the Thorium Committee (2008), 232Th in the Fen Complex in Norway has been considered as a very promising resource, which provides the opportunity for nuclear energy production in the future. However, the available
232Th resources have not been properly characterized; the quoted US GS (2007) weight estimates of the 232Th resources (170 000 tonnes), dating from the 1950s- 1960s, are unreliable (as explained in the previous section). Need for further investigation of 232Th in the Fen Complex bedrock has been highlighted. In addition, the Fen carbonatite is one of the very few REE mineral resources in Europe, making it a potentially strategically important deposit (Østergaard, 2011).
With respect to both 232Th and REE, the possibilities for mining and exploitation of the Fen Complex bedrock are open for future generations. On the other side, mining of Fe and Nb ores was conducted at several locations in the past, and questions related to possible environmental risk from legacy NORM have already been raised. Thus, considering past and possible future mining and extraction activities, care must be taken to address all environmental issues associated with
radionuclides and trace elements. The present assessment should contribute to understanding of the current radiological exposure situation. Also, it will be valuable in the future if exploitation of Fen resources resumes.
• Investigations in the Fen Complex area have focused on the unique geological formation (Brøgger, 1921; Dahlgren, 1983; Landreth, 1979) and on increased human health risk due to terrestrial gamma radiation and 222Rn inhalation (Smethurst et al., 2006; Solli et al., 1985; Stranden and Strand, 1986; Sundal and Strand, 2004). However, neither investigation of contaminants in different environments compartments nor impact assessment for species other than humans has been performed.
• As of 1 January 2011, a new amendment was enforced whereby radioactive waste and radioactive pollution were integrated in the Norwegian Pollution Control Act from 1981. This means that radioactive waste and radioactive pollution are now regulated under the same legal framework as all other pollutants and hazardous wastes. The amendment established two sets of criteria defining radioactive waste:
lower value radioactive waste, and higher value waste - the most common case when waste must be disposed of in a final repository (Liland et al., 2012).
According to this, the legacy NORM sites in the Fen Complex need to be investigated in order to determine whether screening levels have been exceeded and intervention is required.
Based on literature data on NORM in the Fen Complex area, the main hypotheses of the current work were as follows:
H1. Due to the presence of 232Th-containing minerals in the Fen Complex bedrock,
232Th and associated radionuclides and trace elements are expected to represent an environmental problem in the area.
H2. The mobilization of radionuclides and trace elements from surface rock minerals is expected.
H3. Transfer of mobile and bioavailable radionuclide and trace element species from soil to biota is expected.
H4. The mobility and bioavailability of elements should be enhanced at legacy NORM sites in comparison to undisturbed 232Th-rich sites.
H5. Enhanced radiation doses to biota and humans, due to natural radiation in the bedrock, are expected.
The overall objective of this thesis was to assess possible environmental contamination with radionuclides and associated trace elements in the Fen Complex area, and their impacts on biota and humans.
The assessment was organized in tiers common for environmental risk assessments (ERA). The approach was extended to cover the exposure of humans to outdoor radiation (Figure 3). Thus, the final approach represents an integrated radiological impact assessment.
Figure 3. Assessment tiers in the present study.
According to the working tiers, the following sub-objectives were set to meet the main objective:
1) Problem identification and characterization of the contaminants in different environmental compartments (Papers I and II)
2) Estimation of the transfer and possible bioaccumulation of radionuclides and trace elements in biota (Papers III and IV)
3) Assessment of biota exposure, calculation of dose rates (Papers III and IV) 4) Identification of the main pathways of humans outdoor radiation exposure and
estimation of the radiation doses related to outdoor exposure (Paper V)
Existing exposure situation:
Legacy NORM and undisturbed naturally Th rich sites
Internal and external radiation exposure Humans and biota Different environmental
compartments Multiple contaminants
Biota exposure Bioavailabity Transfer factors Absorbed dose rates
– the ERICA tool
Exposure of humans Outdoor gamma radiation
Radon and thoron Environmental media concentrations of radionuclides
and trace elements
Contamination status
Outdoor radiation exposure doses
for humans
Biota dose rates
General conclusions of integrated impact assessment PLANNING AND PROBLEMS
FORMULATION
MEASUREMENTS AND EXPOSURE ANALYSES
IMPACTS CHARACTERIZATION
2 Thesis background
2.1 Naturally occurring radioactive material (NORM)
Radioactive materials can be divided into two broad categories: naturally occurring radioactive materials (NORM) and man-made materials. Several similar definitions of NORM exist. According to US EPA (2006), NORM is defined as a ˝material which may contain any of the primordial radionuclides or radioactive elements as they occur in nature, such as radium (226Ra), 238U, 232Th and their radioactive progenies that are undisturbed as a result of human activities˝. Human manipulation of NORM for economic ends, such as mining, ore processing, fossil fuel extraction and commercial aviation, leads to what is known as ˝Technologically Enhanced Naturally Occurring Radioactive Materials˝ or TENORM (US EPA, 2000; Vearrier et al., 2009). In recent years, the acronym TENORM is often replaced with ˝industrial NORM˝ (which refers to enhanced radiation as a consequence of different industrial activities), ˝legacy NORM˝ (which refers to enhanced radiation as a consequence of human activities from the past) or simply with ˝NORM waste˝. Still, no clear consensus on the use of these terms exists; similar exposure situations have sometimes been named differently what can even be confusing.
Essentially, exposure to undisturbed NORM is responsible for most of an average person’s annual radiation dose, and therefore, is usually not considered to have any particular health or safety significance. However, as explained above, primordial radionuclides present in the parent materials can become concentrated in wastes during different beneficiation processes. This results in radionuclide concentrations in NORM wastes (e.g., industrial NORM, legacy NORM) that are often orders of magnitude higher than in the parent materials (US EPA 2000). Over time, as potential NORM hazards have been identified, they have increasingly become subject to monitoring and regulation, although legal liability related to NORM waste is loosely defined in most countries (Kosako and Sugiura, 2003). In 1996, the International Agency on Atomic Energy (IAEA) published the Basic Safety Standards (BSS), and although the NORM issue was not considered directly, the exemption levels for each radionuclide, including naturally occurring, were given (IAEA, 1996). Furthermore, during the 1990s and 2000s, international interest in NORM problems increased and several international subgroups and forums were organized (meetings in Australia, Brazil, Malaysia, Vietnam, China, Thailand, etc.). Current recommendations of the International Commission on Radiological Protection (ICRP, 2007) state that control
of the dose of radiation, no matter what the source, is the central aspect of protection of the individual.
Although both 232Th and 238U are alpha emitters and are characterized as radiotoxic, very low doses are actually received from pure 232Th and 238U due to their long half- lives (T1/2). Key dose contributors, in the case of NORM waste, are associated with
238U and 232Th progenies, especially 226Ra, polonium (Po) and lead (Pb) isotopes produced from 222Rn. In terrestrial and aquatic ecosystems, these radionuclides can be transferred from the site of origin by air emissions, leaching, and by dissolving in run- off waters, from the soil and into plants, animals and, finally, through the food chain to man (Apps et al., 1988; Bernhard et al., 1996; Stojanovic et al., 2009; Vandenhove et al., 2006; Vandenhove and Van Hees, 2007). It has also been shown that certain fractions of 238U, 232Th and their progenies can be present as radioactive particles (Salbu et al., 2004). Consequently, the transfer to man could occur via inhalation of particles after resuspension and via dietary intake. The exhalation of Rn isotopes from the ground or surface of building materials and subsequent human inhalation is another significant radiation pathway from NORM waste to humans. Thus, improper disposal of NORM waste and/or re-use of NORM contaminated materials can lead to increased exposure of humans, plants and animals to radiation, in a variety of ways.
In many situations worldwide, residents, authorities and industries need additional information, assistance and guidance in management and protection from various types of NORM waste.
As described in section 1, the Fen Complex contains large quantities of 232Th in rocks and soil in an undisturbed natural state (NORM). On the other side, decommissioned mining and disposal sites represent legacy NORM waste. Although the acronyms NORM and TENORM were used in Papers I, III and V, in the present thesis a more precise description "undisturbed 232Th-rich sites" has been used for NORM in the Fen area, and ˝legacy NORM˝ for waste from the former mining sites. The ICRP (2007) recommendations describe three types of exposure situations: planned, emergency and existing. According to the ICRP (2007) definition (˝existing exposure situations are exposure situations that already exist when a decision on control has to be taken˝), decommissioned Nb and Fe mining sites in the Fen Complex should be regarded as existing exposure situations. Furthermore, with respect to the amendment of the Norwegian Pollution Control Act (2010), appropriate legislative requirements should be applied in situations where NORM has been enhanced due to human activities as of 1st January 2011. A tiered approach has been developed; a set of activity levels that determine the necessary activities and possible handling has been provided (Liland et al., 2012).
2.1.1 Thorium (Th)
The element thorium (Th) was discovered in 1828 by the Swedish chemist Jons Jakob Berzelius (1779-1849) when he examined rock samples from Norway. He named it after Thor, the Nordic God of thunder. Thorium is a naturally-occurring, silvery white, slightly radioactive metal, produced together with U and other elements heavier than Fe in one or more supernovae over 6 billion years ago (World Nuclear Association, 2006). Naturally occurring Th consists 100 % of the isotope 232Th by weight, a parent nuclide of the Th decay series (Figure 4). The average concentration of 232Th in the Earth’s crust is 10 mg/kg, which is about two to four times higher than the concentration of naturally occurring U (UNSCEAR, 2000, 2008a). However,
232Th has a much lower specific activity than 238U due to the difference in their T1/2; the activity concentration (Bq/kg) of the two elements is about the same (Kathren, 1998).
As 232Th undergoes radioactive decay, it emits an alpha particle, with accompanying gamma radiation, and forms 228Ra. The decay process continues, radiation (alpha and beta minus particles and gamma rays) releases, and the formation of new radionuclides continues until stable 208Pb is formed (the decay chain consists of 12 different radionuclides) (Figure 4). The half-life of 232Th (T1/2) is about 14 billion years. In the 232Th decay chain, 228Ra and 228Th have T1/2 = 5.8 and T1/2 =1.9 years, respectively, while the T1/2 of other progenies are short (Choppin et al., 2002).
The major physico-chemical form of 232Th in nature is ion Th+4, i.e., tetravalent (IV) oxidation state with very low solubility (US EPA, 1999). Important factors affecting sorption and dissolution of 232Th are the presence of Fe and manganese (Mn) oxides, organic compounds, ligands such as CO32- and humics, Eh and pH (Guo et al., 2008;
Rachkova et al., 2010; Syed, 1998; Vandenhove et al., 2009a). In fact, 232Th is considered to be one of the least mobile elements in continental weathering (Braun et al., 1998), and its mobility is expected to increase only in extremely acid environments (Piñeiro et al., 2002).
Thorium occurs in several minerals, such as thorite ((Th, U)SiO4), thorianite (ThO2), allanite ((Ce, Ca, La, Y, Th)2(Al, Fe2+, Fe+3)3(SiO4)3(OH)), but the most common is the rare earth-thorium-phosphate mineral, monazite ((Ce, La, Nd, Th, Y)PO4), which contains up to about 1-15 % thorium oxide (ThO2) (Anthony et al., 2000; Choppin et al., 2002). With respect to 232Th abundance in Norway, three main regions are the Fen Complex in Telemark County, the Permian Oslo Province and the Southeast coast region. A series of 232Th bearing minerals has been identified at these sites. The Fen
Complex is considered to be a very promising resource, with 232Th amounting to about 0.1 – 0.4 wt % in rødbergite and rauhaugite rocks (Thorium Committee, 2008).
Figure 4. 232Th and 238U radioactive decay chains (http://world-nuclear.org/).
Beside the erosion and weathering of the minerals in rocks and soils, windblown dust and volcanic eruptions can also release 232Th in the environment (Andersson et al., 1995; Gill and Condomines 1992; Thomas et al., 2002). A number of worldwide locations have been demonstrated to have elevated natural radiation due to high 232Th contents in soil or bedrock. These include coastal sands of the States of Espiritu Santo and Rio de Janeiro in Brazil and the State of Kerala in India, as well as regions in the Guangdong Province in China (Kathren, 1998). Mining of naturally occurring 232Th, coal burning and commercial processing, if not properly controlled, could release
232Th and its progenies into the environment and could therefore be man-made sources of contamination and enhanced radiation levels (Dowdall et al., 2004;
Hutchinson and Toussaint, 1998).
Thorium as nuclear energy source
According to the Thorium Committee (2008), there is a prevailing belief that the current model for the world’s energy policy is not sustainable. Emission of greenhouse gasses and their impact on climate, as well as increasing energy demands
worldwide and the potential for secure energy supplies are among the main reasons for this view. The outlook for nuclear energy power has generally improved worldwide, with a progressive improvement in the operating performance of the existing reactors which ensure economic competitiveness of nuclear electricity in world markets. In June 2009, some of 436 nuclear power plants were in operation worldwide, generating about 16 % of global electricity, while an additional 45 power reactors were under construction (Adamantiades and Kessides, 2009).
Over the last 50 years, there has been increasing interest in the utilization of 232Th as nuclear fuel. Currently, the highest research activity on 232Th as a nuclear energy source is in India, where the utilization of 232Th for large scale energy production is the major goal of the nuclear power program (Thorium Committee, 2008). All of the mined 232Th is potentially useable in a reactor, compared with the 0.7 % of natural U (235U); therefore, about 40 times more energy per unit mass is theoretically available from 232Th. The isotope 232Th is not fissile, which means it cannot undergo fission if bombarded by neutrons and, as such, it is not directly usable in a thermal neutron reactor. On the other hand, it is fertile, which means that in combination with fissile
235U or plutonium (239Pu), upon absorption of a neutron, 232Th will transmute to a new fissile material – 233U. In this respect, it is similar to transmutation of 238U to 239Pu in the common U fuel cycle.
The Thorium-Uranium fuel cycle:
232Th + n → 233Th → 233Pa → 233U (fissile) is analogous to the Uranium-Plutonium fuel cycle:
238U + n → 239U → 239Np → 239Pu (fissile)
The potential advantages of a Th-based fuel cycle over an U-based fuel cycle are the greater natural abundance of Th, no need for isotope separation (since mined Th consists of a single 232Th isotope), superior thermo-physical and nuclear properties of ThO2 as compared to UO2, a longer fuel cycle and higher burn up, better resistance to the proliferation of nuclear weapons (due to formation of 233U that emits strong gamma radiation) and little Pu or other transuranics products (Hargraves and Moir, 2011; IAEA, 2005). Still, there are several challenges to the use of Th-based fuel.
Preparation of Th fuel is somewhat more complex (e.g., high sintering temperature, corrosion in reprocessing plants) and more expensive than the preparation of U fuel, as it requires a fissile material (233U, 235U, 239Pu) so that a chain reaction (and thus supply of surplus neutrons) can be maintained. Further, irradiated Th or Th-based fuel contain significant amounts of 233U (T1/2 = 73.64 years) associated with strong gamma
emitting daughters 212Bi and 208Tl. As a result, storage of spent Th-based fuel, as well as necessary remote and automated reprocessing and re-fabrication in shielded cells, increase cost in fuel cycle activities (IAEA, 2005). The feasibility of Th utilization in high temperature gas cooled reactors (HTGR), light water reactors (LWR), pressurized heavy water reactors (PHWR), liquid metal cooled fast breeder reactors (LMFBR) and molten salt breeder reactors (MSBR) was demonstrated. However, thus far Th fuels have not been introduced commercially because the estimated U resources are still sufficient (IAEA, 2005).
From the radiological protection standpoint, to assess doses associated with Th nuclear fuel, the front end (mining, milling, extraction, fuel fabrication) and the back end (operation, waste storage, reprocessing and waste disposal) of the Th cycle need to be considered. Radiation dose contributors from mining, milling, extraction and fuel fabrication are only naturally occurring radionuclides. Although, the parent radionuclides 232Th and 238U are alpha emitters, they are insignificant contributors due to the very long T1/2. The major contributors to increased radiation levels from the
232Th chain are the alpha emitting progenies such as 228Th, 224Ra and short-lived isotopes 216Po,212Po produced from 220Rn (decay chain in Figure 4). Radium is water soluble, mobile and easily transferred from rocks, soil or sediments into the environment. It is often concentrated in ground water. The inert gas 220Rn does not interact with matter, however, it is easily released from rock, soil, water, and building materials. It disintegrates to particulate, reactive 216Po which can then be further mobilized. In this way, 232Th progenies are displaced from the original deposits and contribute to the exposure dose of humans (Thorium Committee, 2008). However, in contrast to 222Rn and its progenies, 220Rn and its progenies are less significant in terms of dose contribution. Due to a short T1/2 of 220Rn (T1/2 = 55.6 sec), it disintegrates to particulate 216Po before reaching some distance. Thus, it can contribute to the exposure dose only if a person is directly exposed near the source. In contrast, the potential of 222Rn (T1/2 = 3.8 days) migration before decay is much bigger, and it represents a significant problem at many worldwide locations (UNSCEAR, 2006).
Furthermore, the T1/2 of the 220Rn progenies (Figure 4) are short (shorter than 222Rn progenies), and consequently their concentrations in the environment and contributions to the exposure dose are low. Due to its short T1/2, only traces of high energy gamma emitting thallium (208Tl) will be present per kg of 232Th. Therefore, the impact of 232Th progenies is much lower and easier to handle than the impact of 238U progenies. The doses received at close distance to the source, during Th mining and milling operations, are comparable to the doses received during U mining, while doses at larger distances (associated with crushed rocks and at tailings) are lower than
